Hydrometallurgy is the art of recovering metals from ores by first effecting solution of the metals in the form of a salt, separating the solution from the impoverished solid, then decomposing the metallic salt in such a way as to cause precipitation of the metal in a state of comparative purity. The leaching solution or lixiviant for reclaiming gold and silver from a bed of low-grade ore generally constitutes aqueous solutions of sodium cyanide mixed with oxygen (air) to convert the noble metal (M) to soluble NaM(CN).sub.2, from which M can be recovered either by precipitation with zinc dust or aluminum powder, carbon absorption, or by electrowinning. Sulfuric acid or sulfuric acid-ferric sulfate is the principle practical copper mineral lixiviant.
The term "heap," as used in the art of hydrometallurgy, means a bed of run-of-mine or granular low-grade ore that has been suitably spread over a prepared surface or "pad" that will ensure solution recovery. The pad is formed as an impervious base, such as by sheets of plastic film (e.g., polyethylene), asphalt and/or compacted clay. The ore is normally heaped onto each impervious pad to a depth of ten to thirty feet, after the ore has been pre-crushed to sufficiently small size to enable the leaching solution to reach the metallic-mineral particles contained in the ore. Gold and silver ores, for example, are run-of-mine or more commonly crushed to 40-200 mesh size with recovery increasing with the increased fineness of the crushed ore. The pad is usually sloped with the pregnant solution recovered for subsequent processing by perforated drain pipes or channels positioned on the impervious base, at the "toe" of the pad.
Lixiviation is the process effecting contact between the ore and the leaching solution. The process is normally accomplished by circulating the solution through the stationary ore mass, commonly known as "percolation." The rate of percolation will depend on a number of factors, such as the particle size of the ore, the depth of the pad, the strength of the leaching solution (e.g., one to four pounds of sodium cyanide per ton of water for recovering gold) and the quantity of leaching solution distributed over the bed of crushed ore per unit area.
Conventional percolation systems for heap leach mining normally include sprayers mounted above the bed of crushed ore which function to spray the leaching solution onto the ore in a non-uniform and uncontrolled manner. The leaching solution is thus prone to rapid evaporation and degradation by its exposure to ultra-violet rays, requiring constant monitoring and replenishing of the leaching solution with added water and/or sodium cyanide. Evaporation is further increased by exposure of the leaching solution to wind friction when it is sprayed through the air.
Surface spraying also induces ice build-up over the pad in freezing weather which in many instances requires complete operation shut-down or a risk of wash-out or blow-out, as will be appreciated by those skilled in the heap leach mining arts. As an alternative to shutting down the operation, the leaching solution and/or the ore itself may be heated, but at substantial cost. The aforementioned wind factor may give rise to an unsafe working environment for the operating and maintenance personnel and also tends to contaminate the surrounding environment (air, ground water, soil) with the highly toxic leaching solution.
Further disadvantages of conventional surface spraying techniques include the formation of surface ponding and run-off, giving rise to serious channeling problems and potential for blow-outs. Ice melt-downs, ponding and other adverse effects, resulting from uneven solution distribution common to surface spraying, will result in pad channeling. The fines washed-down by solution channels form hard pans that eventually divert and internally erode the ore bed and pad. Should these conditions persist, pad blow-out will oftentimes result.
The sprayers used for surface spraying also require constant repair to unplug their spraying heads. Conventional systems of this type are also unusable when the pad is situated over uneven terrain, due to the above-mentioned channeling and pad blow-out problems.